Changes in carbon pools of peatland and forests in northwestern Russia during the Holocene

Global and Planetary Change 16–17 Ž1998. 75–84

Changes in carbon pools of peatland and forests in northwestern Russia during the Holocene K.I. Kobak ) , N.Yu. Kondrasheva, I.E. Turchinovich Dept. of Climatology, State Hydrological Institute, St. Petersburg, 199053, Russian Federation Received 19 December 1995; accepted 1 July 1997

Abstract Estimates of changes in carbon pools of vegetation and soils during the Holocene are contradictory due in part to incomplete information. The peat carbon pool has increased in northwestern Russia due to paludification but the intensity of paludification and carbon accumulation rate was not constant during the Holocene. We estimate that the average long-term accumulation rate in peatlands of Russia was 44 P 10 9 kg Cryr or 28.6 P 10y3 kg Crm2 yr. According to our model calculations, the maximum rate of peat accumulation in northwestern Russia was reached in the Boreal Žearly Holocene. Ž34.5 P 10y3 kg Crm2 yr. and the late Atlantic time Žmid Holocene. Žabout 30 P 10y3 kg Crm2 yr.. The rate decreased during the Subboreal time to 17 P 10y3 kg Crm2 yr. Modern rates of carbon accumulation in some types of peatlands of Russia, calculated by peat-growth model, range from 20 P 10y3 kg Crm2 yr to 100 P 10y3 kg Crm2 yr. The forest composition of northwestern Russia also changed during the Holocene, but the changes in carbon pool of phytomass did not exceed 5% from the mid-Holocene to the present. q 1998 Elsevier Science B.V. All rights reserved. Keywords: North-western Russia; peatland; forest; carbon accumulation; Holocene

1. Introduction The change in atmospheric CO 2 concentration in the late Pleistocene can be reliably determined by the analysis of air bubbles trapped in polar ice. The ice core record from Antarctic stations Že.g., Byrd station. shows that CO 2 concentrations increased from 200 ppmv 17,000–18,000 years ago. Over this period 170 P 10 12 kg of carbon was accumulated in the atmosphere, the average accumulation rate equalling 0.02–0.03 P 10 12 kg of carbon per year ŽSundquist, 1993.. )

Corresponding author.

The oceans are believed to play a major role in the atmospheric CO 2 concentration change due to changes in ocean biology and circulation. The orbital insolation variations in the Northern Hemisphere were probably the main triggers of changes in circulation, redistribution of nutrients to deep water and productivity of the oceans. The role of continental biota in atmospheric CO 2 fluctuations over that interval is still not clear. Some attempts have been made to estimate carbon pools of vegetation and soils during the Last Glacial Maximum and deglaciation ŽPrentice and Fung, 1990; Faure, 1990; Adams et al., 1990; Prentice, 1992; Olson et al., 1985.. The results obtained are highly

0921-8181r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 8 1 8 1 Ž 9 8 . 0 0 0 1 1 - 3

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K.I. Kobak et al.r Global and Planetary Change 16–17 (1998) 75–84

contradictory. According to Prentice and Fung Ž1990., carbon pool of continental biota 18,000 kyr ago was about the same as at the present time, changing insignificantly. According to other estimates ŽFaure, 1990; Adams et al., 1990., vegetation and soils contained much less carbon 18,000 kyr ago, carbon pool increased by about two times by the mid-to-late Holocene ŽTable 1.. Atmospheric and oceanic shifts in 13 Cr 12 C ratios have been documented in ice core and marine sediment records ŽCurry et al., 1988.. They suggest that during deglaciation 450–750 P 10 12 kg of carbon were transferred from the oceans and atmosphere to terrestrial plants and soils and nearshore organic sediments ŽCrowley, 1991; Sundquist, 1993.. Terrestrial plants and soils were probably a net-sink of atmospheric CO 2 at that time but it is very difficult to estimate the intensity of this flux. Estimates of the carbon content in various marine and terrestrial ecosystems and those of fluxes between different pools are given in Fig. 1 and Table 1. There are some components of carbon cycle which have not been studied well enough until now Žmainly biotic components.. In recent years, many attempts have been made to determine carbon content in the phytomass of ecosystems of the former Soviet Union and Russia ŽTable 2.. The obtained estimates vary considerably. These discrepancies result from different methods

and initial materials used. The estimates based on the data from forest inventories Žstatistical data. seem to be more reliable. As Russia occupies a very large territory, corrections made in the estimates of carbon content in ecosystems of the Russian Federation can serve as a basis for reconsidering global estimates of carbon pool of terrestrial ecosystems. The main purpose of the present investigation is to estimate carbon accumulation rate in the peatlands of Russia currently and in the past, which makes it possible to reveal changes in carbon pools due to climate change as well as to predict the response of peatlands to the anticipated climate change.

2. Methodology 2.1. Estimates of aÕerage long-term peat accumulation rate To estimate the average long-term peat accumulation rate we need the data on the areas occupied with various types of peatlands, mean peat bulk density, carbon content in the peat, mean depth of peat layer, and basal age of peat. Recently, the estimates of the areas, occupied with various types of peatlands, have been elaborated for most of European Russia, Western and Eastern Siberia ŽBotch et al., 1995; Vompersky, 1994.. A

Table 1 Approximate content of the carbon reservoirs of the earth system Reservoir

Atmosphere Terrestrial biota Organic carbon in soils Žincluding detritus. Marine biota Dead marine organic matter a Oceans Žnon-organic carbon. Organic carbon rocks Carbonate rocks

Carbon amount, Pg Ž1 Pg s 10 12 kg. Present b

6 kyr c

18 kyr d

750 560 2100 72 2 1830 35,900 12,000,000 94,000,000

660 850 1700–2000 100 -1 675–1000 38,000 10,300–15,600 62,200,000

420 343 625

a This marine carbon pool is poorly understood, with recent observations by Druffel et al. Ž1989. suggesting that it is larger and has more variable lifetimes than previously thought ŽToggweiler, 1990.. b From Kobak, 1988; Prospects for future climate, 1990. c Olson et al., 1985. d Adams et al., 1990; Sundquist, 1993.

K.I. Kobak et al.r Global and Planetary Change 16–17 (1998) 75–84

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Fig. 1. Diagram depicting the reservoirs and fluxes composing the global carbon cycle Žadapted from Kobak, 1988.. The estimated values of carbon storage in the major reservoirs Žin Pg C, 1 Pg s 10 12 kg. and the estimated fluxes of carbon Ž F in Pg Cryr. are indicated as well as the average lifetime of carbon in these reservoirs Žt , in years.. The lifetime given here apply to individual molecules rather than to changes in the net amount in the reservoir. The various fluxes are: F1 -net assimilation of CO 2 in photosynthesis by continental plants; F2 -root respiration; F3-carbon input from litter fall onto the soil surface; F4 -mineralization of litter; F5 -new formation of humus; F6 -stable humus formation; F7 , F8-oxidation of humus; F9-terragenic transfer of organic matter from land to ocean by suspension and windblown dust; F10 -transfer of organic carbon ŽC org . by river runoff; F11-photosynthesis by phytoplankton; F12-mineralization of C org ; F13-transfer to reservoir of particulate and dissolved C org ; F14-deposition of C org on the ocean floor; F15-mineralization of C org on the ocean floor; F16 -transfer of C org to the deep lithosphere; F17 ŽB.-net exchange of CO 2 between the atmosphere and ocean; F18 -mineralization of C org on the coastal shelf and elsewhere; M w -total amount of inorganic carbon in the ocean waters; tml-mixed layer temperature; to -outcrop region temperature; tdo -deep ocean temperature; a F11-flux of new soft organic tissues from the ocean mixed layer to the deep ocean.

significant contribution has been made by the specialists from the State Hydrological Institute, who have been conducting field investigations for several decades. As a result, many discrepancies concerning the estimates of current peatlands areas in Russia

have been removed ŽState Hydrological Institute, 1995.. Peat density varies with depth profile. However, more important are the differences in bulk density between three types of peat: moss Ž80 kgrm3 ., herb

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Reference

Approach used

Carbon content FSU

Kolchugina and Vinson, 1991 Kobak and Kondrasheva, 1991 Kolchugina and Vinson, 1993

Issaev et al., 1993 Krankina and Dixon, 1994 Alekseev and Berdsi, 1994

Kobak and Kondrasheva, 1994

Map-based approach, phytomass of all ecosystems Map-based approach, phytomass of all ecosystems Map-based approach total phytomass living phytomass Forest statistical data, phytomass of all ecosystems Forest statistical data, living phytomass Forest statistical data forest ecosystems non-forest ecosystems Forest statistical data, forest ecosystems, phytomass of living trees

Russia

Total, 10 12 kg

kgrm2

Total, 10 12 kg

kgrm2

91.0 73.8

y 3.83

y y

y y

68.7 50.4 y y

y y y y

y y 41.2 47.1

y y 4.66 y

y y y

y y y

28.0 0.7 25.0

3.63 0.64 3.24

K.I. Kobak et al.r Global and Planetary Change 16–17 (1998) 75–84

Table 2 The carbon content in phytomass of ecosystems of FSU and Russia

K.I. Kobak et al.r Global and Planetary Change 16–17 (1998) 75–84

Ž140 kgrm3 . and wood Ž200 kgrm3 . ŽMacFarlane, 1969; Zoltai et al., 1988; Tjuremnov, 1976.. These values were used while estimating the average longterm carbon accumulation rate ŽBotch et al., 1995.. Carbon content varies, depending on the peat type, from 50% to 60% of dry organic matter of peat ŽLishtvan and Korol, 1975.. We used the available materials on radiocarbon dates of peat deposits and voluminous data on average peat depth ŽYelina et al., 1984; Kuzmin, 1993, etc... The main sources of uncertainty in the estimates presented herein are associated with the areal extent of peatlands in Russia and depth of peat deposits. These sources of uncertainty overwhelm the variations in bulk density and carbon content of peat. 2.2. Current rate of carbon accumulation in peatlands. The estimate of current rate of net carbon accumulation Žlinear approach. in various types of peatlands can be obtained by a well-known model for peat growth ŽClymo, 1984.. Any peatland is described as consisting of two layers; the upper thin layer, acrotelm, and the lower one, catotelm ŽFig. 2.. The boundary between these layers is determined by the long-term minimum position of the peatland water table. All processes in the catotelm Že.g., anaerobic peat decomposition. are very slow and take much more time than in the acrotelm, thousands

Fig. 2. Model of peat growth. Pa-net-productivity; Pc-organic matter flux into the catotelm; Pa eya at-rate of organic matter loss in the acrotelm; Pc eya at-rate of organic matter loss in the catotelm; h a , h c-depth of the acrotelm and the catotelm, respectively.

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of years ŽClymo, 1984.. Changes in peat accumulation with global climate warming can be assessed using model estimates of current carbon accumulation rate for various types of peatlands and establishing the relationship between the carbon accumulation rate and climatic parameters. We used the parameters characterizing the surface layer of peatlands: phytomass productivity, thickness of the acrotelm and its bulk density for the evaluation of the decay constant in the acrotelm and the flux of organic matter to the catotelm. If we consider time intervals no longer than 1000–2000 years, a linear approach to peat accumulation is valid and, using published data on the dated profiles of peat as well as data on its bulk density for various types of peat, linear peat accumulation can be estimated. 2.3. Model estimates of long-term carbon accumulation in peatlands for different periods of the Holocene. For the time intervals longer than 1000–2000 years, the rate of organic matter decomposition in the catotelm should be taken into account. For our preliminary model estimates we used the observed data on 14 C peat age profiles and the published model estimates for the decomposition constant in the catotelm. Most reliable data on the dated peat profiles were compiled for the wetlands in Karelia ŽYelina et al., 1984. and north-western Russian oligotrophic bogs ŽKuzmin, 1993.. Recent model estimates of the decomposition constant in the catotelm published by Tolonen Ž1987., supplemented with our estimates, were used for the evaluation of the carbon accumulation rate in different periods of the Holocene. Assuming that the mean carbon content in peat is equal to 51.7% of the total dry mass ŽGorham, 1991., the phytomass productivity in dry organic matter ranges from 0.5–0.65 kgrm2 yr for the late Holocene to 0.9–1.0 kgrm2 yr for the early and mid-Holocene ŽBazilevitch, 1993; Yelina et al., 1984.. 2.4. Maps of palaeobotanical reconstructions They were used to determine the movement of forest zone with the climate fluctuations, based on

K.I. Kobak et al.r Global and Planetary Change 16–17 (1998) 75–84

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known relationships between forest zone distribution and climate factors at present and with a projected temperature increase by 18 and 28C ŽKobak and Kondrasheva, 1992..

3. Results To determine the current areas occupied by wetlands of various types, the classification by Botch and Masing Ž1983. was used. Seven types of wetlands were identified. Polygonal mires are located in Arctic and Subarctic zones Žtundra, forest-tundra.. Palsa mires are located in tundra, forest-tundra and northern taiga. Aapa mires can be found in northern taiga in European Russia and in Kamchatka. In northern, middle, and southern taiga, raised string, sphagnum and blanket bogs are located. Pine bogs, alder swamps and fens can be found mainly in mixed and broad-leaved forest zones. Reed and sedge fens are located mainly in forest-steppe and steppe zones. Marshes are situated in semidesert and desert zones. More detailed description of the types of wetlands is given in Botch and Masing Ž1983. and Botch et al. Ž1995.. According to our estimates, peatland area in Russia equals 1.54 million km2 and the average long term carbon accumulation in peatlands of Russia was estimated as 44 P 10 9 kg C per year ŽTable 3.. Vertical peat accumulation increases from the subarctic to boreal and temperate zones and reaches the greatest value in the hardwood forest zone Že.g., alder

swamps, fens, etc... Polygonal mires occupy a vast area. However, their peat carbon store is only 6% of the total carbon pool because of their shallow depth. They contribute about 8% to the total annual peat production. The average depth of peat is similar in palsa mires and aapa mires, but palsa mires occupy an area six times greater than aapa mires. The average annual rate of peat accumulation in palsa and aapa mires is 0.2 and 0.5 mm per year, respectively. Palsa mires contribute 10.4% of the total annual peat carbon accumulation of Russia and aapa mires contribute only 4.3% of the total. Raised string bogs account for approximately one-half of the peatland area in Russia where the average long-term rate of peat accumulation is greater than in polygonal, palsa and aapa mires. Raised string bogs contribute 46.8% of the total annual peat production. The long-term rate of peat accumulation in pine bogs, alder swamps, fens and marshes is 0.3 to 1.0 mm per year and they contribute 6.3 to 12.6% to the annual carbon accumulation in Russia. The long-term average rate of peat accumulation in Russia was estimated as approximately 28.6 P 10y3 kg Crm2 yr. It varies from 11.4 P 10y3 kg Crm2 yr Žpolygonal mires. to 50–75 P 10y3 kg Crm2 yr Žmarshes, fens and black alder swamps.. According to Vompersky Ž1994., the long-term average rate of carbon accumulation in peatlands is 22.4 P 10y3 kg Crm2 yr or 38 P 10 9 kg C per year. Modern rate Žlinear approach. of carbon accumulation in some types of peatland in Russia, calculated

Table 3 Long-term accumulation rates for major peatland types of the Russian Federation Peatland type

Area 4

10 km Polygonal mires Palsa mires Aapa mires Raised string bogs Pine bogs and black alder swamps Fens Marshes Total

Dry bulk density 2

%

Carbon content of dry peat

Peat accumulation

Peat carbon accumulation

10 kg m

%

mmryr

10 9 kg Cryr

3

y3

%

30.00 30.00 5.00 67.35 10.80

19.5 19.5 3.2 43.7 7.1

0.14 0.14 0.14 0.08 0.14

54.5 54.5 54.5 54.5 54.5

0.15 0.20 0.50 0.70 0.67

3.43 4.58 1.91 20.56 5.52

7.8 10.4 4.3 46.8 12.6

6.81 4.04 154.0

4.4 2.6 100

0.14 0.14

54.5 54.5

1.00 0.90

5.20 2.77 43.97

11.8 6.3 100.0

Table 4 Model estimates of modern peat growth rate in some types of peatlands in Russia Area Ž%.

Phytomass productivity, kg my2 yry1 in dry organic matter

Dry bulk density Žkg my3 .

Acrotelm thickness Žm.

Decay constant Žyry1 .

Flux to the Catotelm, kg my2 yry1 in dry organic matter

Peat Accumulation, Žmm yry1 .

Polygonal mires Aapa mires Raised string bogs

19.5 3.2 42

0.59 0.31–0.54 0.43–0.52

70–100 80–90 30–50

0.35–0.4 0.1–0.3 observed, 0.38–0.44; calculated 0.42–0.49 observed, 0.49–0.54; calculated, 0.47–0.58

0.015–0.026 0.02–0.06 0.02–0.05

0.031 0.058 0.07

0.22 0.46–0.53 0.88–0.93

0.063

0.79–0.84

0.079 0.103 0.021

1.00–1.10 0.69–0.94 0.13–0.2

Pine bogs European region W. Siberian region Fens Alder swamps

3.6

3.6

30–50 0.30–0.63 0.21–0.63 0.74 0.72–0.78

100–110 140

0.49 0.85

0.01–0.04 0.01–0.04 0.014 0.0076–0.0090

Table 5 Peat accumulation rate during the Holocene Ž10y3 kg Crm2 yr. Holocene epochs

BO, 9000–7800 years ago

AT, 7800–4900 years ago

SB, 4900–2500 years ago

SA, 2500–present time

Peatlands of Karelia Oligotrophic peatlands of NW Russia

34.5 45

27 45

17 43

9–10 18

K.I. Kobak et al.r Global and Planetary Change 16–17 (1998) 75–84

Peatland type

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K.I. Kobak et al.r Global and Planetary Change 16–17 (1998) 75–84

by Clymo peat growth model are presented in Table 4. To obtain these estimates, we used the data on peatland areas and dry bulk density in the catotelm from Table 3, carbon content from Gorham Ž1991., net phytomass productivity from Yelina et al. Ž1984., and Bazilevitch Ž1993.. These data were supplemented with the data on the rate of organic matter decomposition from Kozlovskaya et al. Ž1978., and dry bulk density in the acrotelm and its depth from Ivanov and Novikov Ž1976.. Our estimates are in satisfactory agreement with those by Yelina et al. Ž1984. for the peatlands of Karelia Ž61–678N, 30– 378E.. During the Holocene the rate of peat accumulation varied. Our estimates for Karelian wetlands are as follows: the rate of maximum peat accumulation occurred during the Boreal period Žmean value for the time interval from 9000–8000 years ago is 34.5 P 10y3 kg Crm2 yr. and in the late Atlantic period Žabout 30 P 10y3 kg Crm2 yr.. During the Subboreal period Ž4900–2500 years ago. the rate of peat accumulation gradually decreased Žwith some exceptions. to 17 P 10y3 kg Crm2 yr. The lowest rate of accumulation is in the range of 9–10 P 10y3 kg Crm2 yr during the Subatlantic period Ž2500 years ago–present time.. For oligotrophic bogs of northwestern Russia, the rates of peat growth remained high during the whole Holocene ranging from 43–45 P 10y3 kg Crm2 yr. Estimates of peat growth for different periods of the Holocene presented in this study are somewhat higher for the Holocene and lower than the present rate of accumulation as compared to the estimates by Gorham Ž1991.. Possibly, this results from the fact that the productivity for Karelian wetlands in Yelina et al. Ž1984. is higher than that obtained by other specialists Žcf. Bazilevitch, 1993. for the same type of wetland. There is an agreement between the results obtained by linear interpolation presented in Yelina et al. Ž1984. and those in this study. According to the estimates of the rate of peat accumulation for the Holocene, there exists the correlation between the rate of peat growth and climate change. Apparently, the rate of peat accumulation decreased in the Subboreal period due to sharp cooling and precipitation decrease. During the Boreal warming, from about 9000–8500 years ago, mean temperatures in high latitudes increased by 4–58C as

compared to the temperatures in the previous period and there was an increase in precipitation by 50 mm ŽBorzenkova, 1992..

4. Discussion From paleodata on temperature and precipitation and estimates of the rate of peat growth, it is obvious that during warm and wet periods of the past, the peat growth accumulation was higher than that in the cool and dry periods ŽYelina et al., 1984; Table 5, the present paper.. Despite the fact that peat accumulation rate varied over the Holocene, the peatland carbon pool was continuously increasing. This increase resulted from vertical growth of peat layer, peatland transgression, and the formation of new peatlands. According to the estimate by Neustadt Ž1984., average paludification rate on the FSU territory over the Holocene equalled 150 km2ryear. This estimate was obtained as a result of investigation of Chistik bog ŽCentral European Russia. with the area equal to 80 km2 . For Karelia, average peatland area growth rate over the Holocene was 4 km2ryear, ranging from 2–7.5 km2ryear ŽYelina et al., 1984.. Using the data on the mean paludification rate for Karelia over the Holocene and peatland area in the FSU, presented in Botch et al. Ž1995., we calculated the average paludification rate for the FSU territory over the Holocene to be equal to 180 km2ryear, this value being in good agreement with the estimate by Neustadt Ž1984.. Paludification process was most rapid in Boreal and Atlantic times of the Holocene and much more less in Subboreal time, when the climate of Karelia was cooler and dryer. During the late Holocene the paludification of Karelia became a bit higher. Paludification of forests on the plains of northwestern Russia is a widely spread phenomenon. Many specialists think that forests and peatlands are the main components of the natural post-glacial plain landscape under moderately humid climate conditions ŽIvanov and Shumkova, 1967; Pjavchenko, 1967, 1985; Zoltai et al., 1988.. Forest ecosystems of northwestern Russia changed significantly during the Holocene. Birch woodland and birch forests of the earlier Boreal period were changed to mixed pine-birch forests by the end of this period. The Atlantic time was characterized by

K.I. Kobak et al.r Global and Planetary Change 16–17 (1998) 75–84

pine, birch-pine and spruce forest mixed with broadleaved trees. The boundary between middle and southern taiga was located 500 km farther north than at present. During the Subboreal period, the role of broad-leaved trees decreased and the middle-southern taiga boundary shifted southward by 150 km. During Subatlantic period, the vegetation in Karelia was similar to the present ŽYelina et al., 1984.. Despite the successions, carbon pool of forest phytomass changed relatively little. According to our estimates, in the Holocene optimum Žthe second Late Atlantic maximum, 6200–5300 before present. it differed by not more than 5% from the current one ŽKobak and Kondrasheva, 1992.. This is much less than the change in Siberian forest phytomass from the middle Holocene Ž4600–6000 years before present. to the present time Ž19.1 P 10 12 kg or 20%., which occurred due to the change of shade-tolerant dark-needled taiga for light-demanding light-needled species ŽMonserud et al., 1995.. Current global climate warming could markedly affect the biosphere ŽHoughton et al., 1990; Budyko et al., 1990.. Noticeable changes in the location and areas of forest zones are likely to occur with the global warming by 1–28C. Modern tundra is expected to be predominantly replaced by coniferous forest. Mixed forests would also shift northward by about 10 degrees latitude. Broad-leaved forests are expected to increase in area, extending both northward and eastward ŽKobak and Kondrasheva, 1992.. The northward expansion of forest ecosystems can be limited by the peat-forming paludification process. The forests in north-western Russia are likely to continue being paludified, that is, replaced by peatlands. On the plains, paludification processes are most intense at the peatland margins. For the next decade the rate increase in peatland area is expected to be equal to that of the second half of the Atlantic period, i.e., about 7000 km2 per 1000 years. The peat depth is expected to increase by 1 cm for the next decade. Future rates of peat accumulation might be higher than the present ones, a problem that should be investigated thoroughly. 5. Conclusion During the Holocene, peatland areas increased in extent in northwestern Russia. The average rate of

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this increase was about 4 km2 per year, ranging from 2–7.5 km2 per year ŽYelina et al., 1984.. The current net carbon accumulation rate equals 0.02–0.1 kg my2 yry1 for the peatlands of Russia, and the average value for the Holocene is 44 P 10 9 kg Cryear. The progressive increase in peatland area in the future is likely to cause increase in CO 2 net-sink from the atmosphere as unmined peatlands can store carbon for thousands of years. The rate of such net sink increase is very small. However, it should be taken into account while planning longer-term strategies for mitigating the unfavourable consequences of global warming.

Acknowledgements The authors thank Prof. H. Faure for comments and the opportunity to present the results of the research at the XIV INQUA Congress. They are grateful to S. Zoltai and an anonymous referee for revision of the manuscript. The research has been funded in part by Grant No. NT 1000 from the International Science Foundation. A grant from INTAS Project No. 93-2037. Žfunded by the European Community. is gratefully acknowledged. This a contribution to INQUA Commission on Terrestrial Carbon and to IGCP Project No. 404 ŽIUGS-UNESCO..

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